1. Field of the Invention
This invention is generally related to safety valves. More particularly, this invention pertains to subsurface safety valves which employ a curved flapper for controlling fluid flow through a production tubing string.
2. Description of the Related Art
Surface-controlled, subsurface safety valves (SCSSVs) are commonly used to shut in oil and gas wells. Such SCSSVs are typically fitted into a production tubing in a hydrocarbon producing well, and operate to block the flow of formation fluid upwardly through the production tubing should a failure or hazardous condition occur at the well surface.
SCSSVs are typically configured as rigidly connected to the production tubing (tubing retrievable), or may be installed and retrieved by wireline, without disturbing the production tubing (wireline retrievable). During normal production, the subsurface safety valve is maintained in an open position by the application of hydraulic fluid pressure transmitted to an actuating mechanism. The hydraulic pressure is commonly supplied to the SCSSV through a control line which resides within the annulus between the production tubing and a well casing. The SCSSV provides automatic shutoff of production flow in response to one or more well safety conditions that can be sensed and/or indicated at the surface. Examples of such conditions include a fire on the platform, a high/low flow line pressure condition, a high/low flow line temperature condition, and operator override. These and other conditions produce a loss of hydraulic pressure in the control line, thereby causing the flapper to close so as to block the flow of production fluids up the tubing.
Most surface controlled subsurface safety valves are “normally closed” valves, i.e., the valves utilize a flapper type closure mechanism biased in its closed position. In many commercially available valve systems, the bias is overcome by longitudinal movement of a hydraulic actuator. In some cases the actuator of the SCSSV includes a concentric annular piston. Most commonly, the actuator includes a small diameter rod piston, located in a housing wall of the SCSSV.
During well production, the flapper is maintained in the open position by a flow tube down hole to the actuator. From a reservoir, a pump at the surface delivers regulated hydraulic fluid under pressure to the actuator through a control conduit, or control line. Hydraulic fluid is pumped into a variable volume pressure chamber (or cylinder) and acts against a seal area on the piston. The piston, in turn, acts against the flow tube to selectively open the flapper member in the valve. Any loss of hydraulic pressure in the control line causes the piston and actuated flow tube to retract, which causes the SCSSV to return to its normally closed position by a return means. The return means serves as the biasing member, and typically defines a powerful spring and/or gas charge. The flapper is then rotated about a hinge pin to the valve closed position by the return means, i.e., a torsion spring, and in response to upwardly flowing formation fluid.
In some wells, high fluid flow rates of as much as 250 million cubic feet or more per day of gas may be produced through the SCSSV. In high flow rate wells, it is well known that curved or arcuate flappers may be used to provide a larger inside diameter, or bore, in the SCSSV as compared to a flat flapper. By design, curved flapper arrangements enable a larger production tubing inner diameter, and thus, allow for a greater rate of hydrocarbon production through the valve area.
In either flat or curved flappers, as the tubular piston and operator tube retract, the flapper closure passes across the lower end of the operator tube and throttles the flow as it rotates toward the closed or “seated” position. At high flow rates, a high differential pressure may be developed across the flapper that may cause distortion and warping of the flapper as it rubs against the operator tube. Also, the flapper may be damaged if it is slammed open against the valve housing or slammed shut against the valve seat in response to the high-pressure differentials and production flow regimes. Deposition of sand particles or other debris on the valve seat and/or sealing surfaces may also cause misalignment of the flapper relative to the valve seat. Such misalignment prevents correct seating and sealing of the flapper. Consequently, a large amount of formation fluid may escape through the damaged valve, wasting valuable hydrocarbon resources, causing environmental pollution, and creating potentially hazardous conditions for well operations personnel. Furthermore, during situations involving damage to the wellhead, the well flow must be shut off completely before repairs can be made and production resumed.
Therefore, a need exists for an improved subsurface safety valve for controlling fluid flow in a well bore.